The present invention relates to conventional fluorescent lamps in which the mercury vapor pressure is controlled by controlling the temperature of the lamps that heretofore have been dosed with liquid mercury, and more particularly to such lamps containing mercury in the form of a zinc amalgam that, in contrast to the predicted equilibrium condition, is in a metastable, non-equilibrium state.
All fluorescent lamps contain mercury which is vaporized during lamp operation. The mercury vapor atoms efficiently convert electrical energy to ultraviolet radiation with a wavelength of 253.7 nm when the mercury vapor pressure is in the range of approximately 2.times.10.sup.-3 to 2.times.10.sup.-2 torr (optimally about 6.times.10.sup.-3 torr). The ultraviolet radiation is in turn absorbed by a phosphor coating on the interior of the lamp wall and converted to visible light. The temperature of the coldest spot on the inner wall of the lamp when the lamp is operating is referred to as the "cold spot temperature" and will determine the mercury vapor pressure within the lamp.
When a lamp containing only mercury operates with a cold spot temperature above about 40.degree. C., the mercury vapor pressure will exceed the optimal value of 6.times.10.sup.-3 torr. As the temperature increases, the mercury vapor pressure increases and more of the ultraviolet radiation is self-absorbed by the mercury, thereby lowering the efficiency of the lamp and reducing light output.
The mercury vapor pressure may be maintained within the desired range either by controlling the cold spot temperature of the lamp (hereinafter referred to as "temperature control") or by introducing other metallic elements into the lamp in the form of amalgams that maintain the mercury vapor pressure (hereinafter referred to as "amalgam control"). For example, fluorescent lamps that have cold spot temperatures above about 75.degree. C., such as some types of small diameter, low wattage fluorescent lamps generally known as "compact" fluorescents, are amalgam controlled in that they typically require two or more elements in addition to mercury which may be introduced into the lamp as solid ternary or multicomponent amalgams. Such amalgam controlled lamps rely on establishment of thermodynamic equilibrium for proper lamp operation (see, for example, U.S. Pat. No. 4,145,634 issued Mar. 20, 1979 to Evans, et al.).
The present invention is directed to temperature controlled fluorescent lamps.
Temperature controlled fluorescent lamps may operate with a cold spot temperature below about 75.degree. C. (typically ranging from 20.degree. to 75.degree. C.) and desirably 40.degree. C. to 60.degree. C. Such lamps are also referred to as "low temperature" fluorescent lamps.
In temperature controlled lamps (e.g., ceiling mounted fluorescent lamps) the mercury is typically introduced into the lamp as a liquid in an amount related to the wattage and rated life of the lamp. For example, 10-15 milligrams of liquid mercury are typically needed to attain an average rated life of 20,000 hours for a 40 watt fluorescent lamp.
However, the high speed, automated manufacturing processes typically used to dose each lamp with liquid mercury lack precision because of the nature of the liquid mercury, the length and configuration of the path by which introduced, and the atomization of the mercury by the high velocity puff of inert gas used to effect introduction. As a result of the variability in the amount of mercury which reaches the lamp, a considerable excess of liquid mercury is used to insure that at least the minimum amount of liquid mercury is introduced into each lamp. Some of the known manufacturing processes allot an average of three to five times the amount of liquid mercury needed to achieve average rated life. Thus, most lamps receive far more mercury than is needed, even up to ten times the amount needed, to achieve the average rated life.
This use of excessive amounts of liquid mercury is wasteful and may produce very unfavorable consequences. For example, only part of the total amount of liquid mercury introduced into the lamp is converted to vapor when the lamp is operating leaving droplets of liquid mercury that cause dark spots on the lamp that are aesthetically undesirable. Further, and perhaps more significantly, mercury is toxic and lamp disposal is becoming a significant issue throughout the world. Thus, it is clearly desirable to manufacture fluorescent lamps with the minimum amount of mercury needed to meet the average rated life.
Accordingly, it is an object of the present invention to obviate many of the above discussed problems and to provide a novel fluorescent lamp which contains a controlled amount of mercury.
It is another object of the present invention to provide a novel temperature controlled fluorescent lamp which contains mercury in the form of a zinc amalgam.
It is yet another object of the present invention to provide a novel fluorescent lamp in which mercury is introduced into the lamp in the form of a solid binary amalgam and which retains most of the second constituent of the binary amalgam (e.g., zinc) in solid form during lamp operation.
It is still another object of the present invention to provide a novel lamp fill material for a temperature controlled fluorescent lamp that is solid and easily handled at temperatures below about 40.degree. C.
It is a further object of the present invention to provide a novel method of introducing a precise amount of mercury into a temperature controlled fluorescent lamp.
It is yet a further object of the present invention to provide a novel method of dosing a fluorescent lamp with a solid, reducing the total mercury by allowing more accurate and reliable dosing.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments.